A limited-slip brake control system is disclosed in copending application Ser. No. 714,425, filed Aug. 13, 1976, entitled "LIMITED-SLIP BRAKE CONTROL SYSTEM," by Robert L. Amberg and Narinder S. Attri, and assigned to the assignee of the present invention (now U.S. Pat. No. 4,078,845, issued Mar. 14, 1978). This brake control system functions to limit the brake force developed by the tires of a vehicle to the nonslip portion of a tire brake force and wheel velocity characterisitic curve, i.e., the front or positive slope side of the characteristic Mu/slip curve for the vehicle, to thereby minimize both tire and brake wear. The system includes three aspects: a basic deceleration control circuit; a large deviation control circuit; and an energy balance system.
The basic deceleration control circuit produces a wheel deceleration signal by filtering and differentiating a signal representing measured wheel speed. The wheel deceleration signal is then compared with a selected reference deceleration signal representing a reference or desired deceleration and a first deceleration error signal is generated which is applied to a hysteresis circuit which outputs a constant level, positive or negative signal. The output signal from the hysteresis circuit is integrated and supplied to a valve drive which in turn controls a brake valve to modulate the metered brake pressure applied to the wheel to be braked. The reference deceleration represented by the selected reference deceleration signal is chosen so that brake force developed as a result of the application of brake pressure is limited to the positive slope or nonslip portion of a characteristic Mu/slip curve for the vehicle, and the basic deceleration control circuit functions to cycle brake pressure, and therefore brake force, about a value that will produce the desired deceleration.
A situation may be encountered, however, where the coefficient of friction between the braked wheel and the ground surface is abruptly lowered, e.g., where a patch of ice exists on a runway. If the reference deceleration has been set at a level corresponding to the brake force that can be developed for a ground surface having a higher coefficient of friction, e.g., a dry runway, then the basic deceleration control circuit will command a brake pressure that will seek to develop a higher braking force than can be obtained when the lower coefficient of friction surface condition is encountered, thus resulting in an actual brake force which lies on the negative slope or slip portion of the Mu/slip curve and which causes the braked wheel to skid. The large deviation control circuit accordingly provides a second deceleration error signal when the wheel deceleration exceeds the reference deceleration by a predetermined amount. This second deceleration error signal is supplied to a lag circuit which is controlled to obtain an output signal which, when supplied to the valve driver, results in brake pressure being removed from the braked wheel in an amount and for a time related to the amount and time that the wheel deceleration exceeds the reference deceleration. Typically, the output signal from the lag circuit in the large deviation control circuit, and the output signal from the hysteresis circuit in the basic deceleration control circuit, are summed at the input to the valve driver so that the large deviation control circuit may control brake pressure when a skid is encountered and so that the basic deceleration control circuit may control brake pressure at all other times.
In order that all brakes of a multiwheeled vehicle perform properly with even wear of the tires of the braked wheels, the energy balance system is used for supplying the same brake pressure to all brakes, such as by using a common brake valve. Alternatively, the brake energy being put into each wheel may be determined by measuring, for each wheel, the actual brake torque and wheel speed, by multiplying the measured brake torque and wheel speed, and by integrating the resultant product. The measured brake energies are then compared, and any difference between brake energies is integrated and used to reduce, through a valve driver for each wheel, the brake pressure applied to the wheels that are working more and to increase the brake pressure applied to the wheels that are working less.
The aforementioned basic deceleration control circuit, large deviation control circuit, and energy balance system are discussed in more detail in the aforementioned U.S. Pat. No. 4,078,845 which is expressly incorporated herein by reference.
The basic object of the present invention is to provide a limited-slip brake control system which incorporates certain improvements over that disclosed in U.S. Pat. No. 4,078,845.
As a first example, the basic deceleration control circuit and the large deviation control circuit disclosed in U.S. Pat. No. 4,078,845 each function to integrate a deceleration error signal obtained from a comparison of wheel deceleration with a reference deceleration. Since the output signals obtained from such integration in the large deviation control circuit and the basic deceleration circuit are summed before application to the valve driver, the integration functions provided in those circuits tend to oppose each other so that a resultant control signal applied to the valve driver does not precisely respond to desired changes in brake pressure commanded by either the large deviation control circuit or the basic deceleration control circuit.
It is therefore an object of the present invention to provide a simpler system than that disclosed in U.S. Pat. No. 4,078,845, which simpler system includes both a large deviation control circuit and a basic deceleration control circuit, and which system permits both of these circuits to more precisely control brake pressure than is possible with the limited-slip brake control system disclosed in U.S. Pat. No. 4,078,845.
As a second example, the limited-slip brake control system in U.S. Pat. No. 4,078,845 is not capable of satisfactorily compensating for a ground surface condition having a low coefficient of friction that is encountered immediately upon application of brake pressure. In particular, the limited-slip brake control system in U.S. Pat. No. 4,078,845 permits brake pressure to build up to the maximum value established by the basic deceleration control circuit upon initial application of brake pressure. If a low coefficient of friction ground surface condition is encountered upon this initial application of brake pressure, the basic deceleration control circuit forces the vehicle into an initial skid, as previously described, which skid is eventually compensated for by the large deviation control circuit. It is desirable in certain circumstances to minimize the effect of this initial skid, particularly in the case where the vehicle is an aircraft and the initial skid occurs immediately upon touchdown.
It is therefore another object of this invention to provide an improved limited-slip brake control system which very quickly reduces brake pressure upon the occurrence of an initial skid.
As a third example, the braked wheels of an aircraft are supported from the aircraft fuselage by a lightly-damped landing gear strut assembly. Application of brake pressure by the limited-slip brake control system in U.S. Pat. No. 4,078,845 results in fore-and-aft oscillation of the landing gear strut assembly as the aircraft touches down and proceeds down the runway. This fore-and-aft oscillation produces an apparent variation in the velocity of the braked wheel which is sensed by the limited-slip brake control system as an apparent variation in wheel deceleration. As a result, the limited-slip brake control system functions to erroneously vary brake pressure about the value otherwise commanded by the basic deceleration control circuit.
It is therefore a further object of this invention to provide an improved limited-slip brake control system which is relatively insensitive to apparent changes in wheel deceleration occasioned by landing gear strut assembly oscillation.